#' @rdname geom_qq
#' @export
#' @param line.p Vector of quantiles to use when fitting the Q-Q line, defaults
#' defaults to `c(.25, .75)`.
#' @param fullrange Should the q-q line span the full range of the plot, or just
#'   the data
geom_qq_line <- function(mapping = NULL,
                         data = NULL,
                         geom = "path",
                         position = "identity",
                         ...,
                         distribution = stats::qnorm,
                         dparams = list(),
                         line.p = c(.25, .75),
                         fullrange = FALSE,
                         na.rm = FALSE,
                         show.legend = NA,
                         inherit.aes = TRUE) {
  layer(
    data = data,
    mapping = mapping,
    stat = StatQqLine,
    geom = geom,
    position = position,
    show.legend = show.legend,
    inherit.aes = inherit.aes,
    params = list(
      distribution = distribution,
      dparams = dparams,
      na.rm = na.rm,
      line.p = line.p,
      fullrange = fullrange,
      ...
    )
  )
}

#' @export
#' @rdname geom_qq
stat_qq_line <- geom_qq_line

#' @rdname ggplot2-ggproto
#' @format NULL
#' @usage NULL
#' @export
StatQqLine <- ggproto("StatQqLine", Stat,
 default_aes = aes(x = stat(x), y = stat(y)),

 required_aes = c("sample"),

 compute_group = function(data,
                          scales,
                          quantiles = NULL,
                          distribution = stats::qnorm,
                          dparams = list(),
                          na.rm = FALSE,
                          line.p = c(.25, .75),
                          fullrange = FALSE) {

   sample <- sort(data$sample)
   n <- length(sample)

   # Compute theoretical quantiles
   if (is.null(quantiles)) {
     quantiles <- stats::ppoints(n)
   } else {
     stopifnot(length(quantiles) == n)
   }

   theoretical <- do.call(
     distribution,
     c(list(p = quote(quantiles)), dparams)
   )

   if (length(line.p) != 2) {
     stop(
       "Cannot fit line quantiles ", line.p,
       ". Parameter line.p must have length 2.",
       call. = FALSE)
   }

   x_coords <- do.call(distribution, c(list(p = line.p), dparams))
   y_coords <- quantile(sample, line.p)
   slope <- diff(y_coords) / diff(x_coords)
   intercept <- y_coords[1L] - slope * x_coords[1L]

   if (fullrange & !is.null(scales$x$dimension)) {
     x <- scales$x$dimension()
   } else {
     x <- range(theoretical)
   }

   new_data_frame(list(x = x, y = slope * x + intercept))
 }
)
